1. Field of the Invention
The present invention relates to an improved helically geared compound mechanical change gear transmission structure, preferably for vehicular compound transmissions of the multiple substantially identical countershaft type. In particular, the present invention relates to an improved, helically geared, compound mechanical transmission structure allowing, for a given transmission capacity and life, the provision of a lighter, axially shorter and/or less costly transmission as compared to comparable prior an transmission structures.
2. Description of the Prior Art
Compound change gear transmissions, usually vehicular transmissions for heavy duty vehicles, of the type having one or more auxiliary sections connected in series with a main transmission section are well known in the prior art. Briefly, by utilizing main and auxiliary transmission sections connected in series, assuming proper sizing of the ratio steps, the total of available transmission ratios is equal to the product of the main and auxiliary section ratios. By way of example, at least in theory, a compound change gear transmission comprising a four (4) forward speed main section connected in series with a three (3) speed auxiliary section will provide twelve (4.times.3=12) available forward ratios.
Auxiliary transmission sections are of three general types: range type, splitter type or combined range/splitter type.
In compound transmissions having a range type auxiliary section, the auxiliary section ratio step or steps are greater than the total ratio coverage of the main transmission section ratios used in both ranges and the main section is shifted progressively through its ratios in each range. Examples of compound transmissions having range type auxiliary sections may be seen by reference to U.S. Pat. Nos. 3,105,395; 2,637,222 and 2,637,221, the disclosures of which are hereby incorporated by reference.
In compound transmissions having a splitter type auxiliary section, the ratio steps of the splitter auxiliary section are less than the ratio steps of the main transmission section and each main section ratio is split, or subdivided, by the splitter section. Examples of compound change gear transmissions having a splitter type auxiliary sections may be seen by reference to U.S. Pat. Nos. 4,290,515; 3,799,002; 4,440,037 and 4,527,447, the disclosures of which are hereby incorporated by reference.
In a combined range and splitter type auxiliary section, or sections, both range and splitter type ratios are provided allowing the main section to be progressively shifted through its ratios in at least two ranges and also allowing one or more of the main section ratios to be split in at least one range.
One example of a compound transmission having a single combined range/splitter type auxiliary section may be seen by reference to U.S. Pat. Nos. 3,283,613; 3,648,546, the disclosures of which are hereby incorporated by reference. Another example is the "Ecosplit" model of transmission sold by Zahnradfabrik Friedrichshafen Aktiengeseushaft of Friedrichshafen, Federal Republich of Germany which utilizes a separate splitter auxiliary section in front of, and a separate range auxiliary section behind, the main transmission section.
Another example of a compound vehicular transmission having a single combined three gear layer, four-speed combined splitter/range transmission may be seen by reference to U.S. Pat. Nos. 4,754,665 and 4,944,197, the disclosures of which are incorporated herein by reference.
It should be noted that the terms main and auxiliary sections are relative and that if the designations of the main and auxiliary sections are reversed, the type of auxiliary section (either range or splitter) will also be reversed. In other words, given what is conventionally considered a four-speed main section with two-speed range type auxiliary section, if the normally designated auxiliary is considered the main section, the normally designated main section would be considered a four-speed splitter type auxiliary section therefor. By generally accepted transmission industry convention, and as used in this description of the invention, the main transmission section of a compound transmission is that section which contains the largest (or at least no less) number of forwards speed ratios, which allows selection of a neutral position, which contains the reverse ratio(s) and/or which is shifted (in manual or semiautomatic transmissions) by manipulation of a shift bar or shift rail or shift shaft/shift finger assembly as opposed to master/slave valve/cylinder arrangements or the like.
The prior an compound change gear transmissions, especially the prior art compound transmissions of the type having both range and splitter type auxiliary section gearing, such as the "Roadranger" type and "Super 10" type offered by Eaton Corporation and the "Ecosplit" type offered by Zahnradfabrik Friedrichshafen Aktiengeseushaft, are well received and widely used in manually shifted heavy duty vehicles. However, these and other types of prior art compound change gear transmissions are not totally satisfactory as it is an ongoing objective, especially for vehicular transmissions, to provide transmissions of equal or improved capacities and reliability which use fewer pans and/or are axially shorter and/or lighter in weight as compared to prior art transmissions of comparable capacity and reliability.